The Science Journal of the American Association for Respiratory Care

2001 OPEN FORUM Abstracts

THENATURE OF UNSTABLE VENTILATORY SUPPORT DURING PSV IN A LUNG MODEL OF OBSTRUCTIVEDISEASE.

LiChen PhD, Alexander Adams MPH RRT, John Hotchkiss MD, Regions Hospital,St. Paul, MN

Background: Previousinvestigations have suggested that pressure support ventilation ( PSV) developsdynamic instability in a model of airway obstruction. To explore the natureof this instability, we investigated the variability of tidal volume (VT)delivery during PSV under combinations of respiratory resistance and complianceunder a range of inspiratory flow cutoff levels.

Methods: A mechanicaltest lung (Hans Rudolph, Kansas City, MO) was used to simulate spontaneouslybreathing patients with combinations of resistance ( R=25, 20, 15, 10 cmH2O/L/s) and compliance ( C=0.12, 0.10, 0.08, 0.06 L/cmH2O), and respiratoryefforts at 20/min. The test lung was ventilated with bilevel pressure supportat 22 cm H2O, PEEP = 5 cm H2O ( model 840, Mallinckrodt, Carlsbad,CA) at inspiratory flow cutoff levels of 45% to 1%.

Results: Thefigure depicts examples of 2 conditions: stable and unstable support conditions.Mean VT ( ± SD) from 20 consecutive breaths is plotted ? dependentupon inspiratory flow cutoff settings. The stable support occurred with lowresistance (R=10)/ moderate compliance (C=.06) and produced a constant frequencyof 20/min. The high resistance (R=25)/ high compliance (C=.12) condition resultedin variable VT and dropped breaths producing an unstable frequency(range of 6.7 to 20/min).

Conclusion: Unstableventilatory support during PSV, a variable VT and a decrease in respiratoryfrequency, can occur at lower inspiratory flow cutoff levels in obstructivedisease conditions. Therefore, optimizing the inspiratory flow cutoff settingmay improve patient-ventilator synchrony.

 

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